You cannot simluate 12-bit in Photoshop. No file formats support 12-bit (to the best of my knowledge). I suggest you build your website in full colours and let the P800 do the translation, it shouldn't be too much of a colour deficit with 4096 colours anyway.

Devices like the P800 or the SPV show colours quite differently than on your computer screen due to the nature of their screen sizes and light conditions etc. I personally wouldn't go thru the trouble of making a 12-bit image because it would as you mentioned be so off anyway.

Introduction
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